1. Field of the Invention
The present invention relates to a frequency measuring device for measuring the frequency of an electric power system.
2. Description of the Related Art
A known frequency measuring device includes a means for sampling an input signal at a predetermined cycle or period to convert it into digital data thereby to obtain a data row, a means for obtaining, from the data row, the timing of each zero crossing point based on pieces of data at a plurality of points before and after each zero crossing point, and a means for obtaining the fundamental wave frequency of an input signal by detecting, as one cycle or period thereof, the time between two adjacent zero crossing points at which the input signal crosses a zero level in the same direction (for instance, see a patent document: Japanese patent application laid-open No. H09-166630).
However, the conventional frequency measuring device as referred to above determines zero crossing timings of the input signal through linear interpolation from a plurality of pieces of data in a sampling cycle or period, so it takes time to perform convergent calculations for linear interpolation, and hence the measurements of the zero cross timings need a relatively long time.
In addition, after a zero crossing point has been detected, the detection of further zero crossing points at a midpoint and its neighborhood of one period at a frequency that has been set in advance as an estimated frequency, i.e., an estimated fundamental wave frequency of an input signal is nullified. As a result, when there are comparatively many harmonic components or noise components, the detection or measurement of zero crossing points would be influenced by such harmonic components or noise components.